Well... Let's try to answer (as I am a newbie :)
I found your question very easy. I am sure I am wrong so I take my camel book to see reverse's section and I seek something which I would not have seen before.
I am not sure that reverse ($aloha) is assumed as a LIST or a string. But my answer is "hello" ! ... and reverse ("$aloha") will return "olleh".
The scalar function forces a scalar context an indeed, the output yields "olleh".
So what is putting reverse($aloha) in a list context? It is our friend, the print function. If you kindly refer to your Camel book, or your Perldoc, or your Learning Perl book or whatever handy refernce you have a available, you will see that print places its arguments into a list context.
So, looking back at our original piece of code, the reason why it prints just plain old "hello", is that it is reversing the elements in the list and not the bytes in the string. Since there is only one element in the list, the list reversed is precisely same as the original list!
Monks, please chime in if this explanation is not exactly accurate.